They've hinted that it's one of the way they're considering making room for monks and/or Panderan.

The going theory is that they'll remove the 10-toon-per-realm limit, but keep the account limit at 50

They've talked about this topic a few times and there has been mention of lifting the realm cap, but keeping the account cap of 50.

This was asked at this year's Blizzcon, and Greg Street replied to it by saying that they are considering it. He mentioned that they might do away with the per-realm limit while keeping the 50-character limit across all realms in place.

So it's definitely within the realm of possibility, and Blizz is very much aware of players' desire for more character slots on a realm.

I wish they would do this before MoP. I'd like it if I could try Horde side and explore their story more deeply on the same realm as my Alliance toons so I could swap the heirlooms back and forth without paying the $25 or $30 (I forget which) it costs to move a toon. I'm at 7 85s, 1 81 (Hunter), 1 30 (Warlock) and 1 15 (Priest) so before long I'll have all my toons stagnating at 85.

And a reminder... At some point last year, there was an EU survey with a question asking if the the player would be willing to pay for additional character slots. It was discussed briefly in a WoW Insider article or two, and I never did see anything further or whether they released the results of the survey.

I would think the percentage of players in dire need of more slots per server is low in comparison to total players, but I'm one of them and would really love to see this in MoP (or sooner). However, I really hope they don't make it a paid service.

I think it's a pretty safe bet that they will do this. My reasoning? The account-wide achievement system they are considering will open the opportunity for achievements like, "Reach level 90 with one of each class." If they're going to have stuff like that, they're going to want to give us the opportunity to work on it all on one server.